Wait Till Next Time

By J. MADELEINE NASH

Human memories are short, and even as the tattered ghost of Hurricane Floyd finally blew itself out over eastern Canada last weekend, it was easy to forget that it began the week as a meteorological giant--one of the century's largest and most powerful Atlantic storms. If it seems as if hurricanes are getting stronger these days, that's because they are. After a 30-year lull, the U.S. is once again being visited by hurricanes the size of the ones that battered the Eastern seaboard in the 1940s, '50s and '60s. Thanks to an unlucky confluence of events--warm Atlantic waters, brisk trade winds and some strange doings in the eastern Pacific--we're on the cusp of what could be an extended spell of very heavy weather.

Floyd is nothing, scientists warn, compared with what may lie ahead. In the next century, they say, we may see hurricanes that far exceed Floyd's top sustained winds and approach a hurricane's upper limit of 180 m.p.h.--more than capable of sending a 30-ft. wall of water surging inland, flattening houses, inundating coastal cities and stirring the ocean bottom to a depth of 600 ft.

Moreover, that 180-m.p.h. speed limit pertains only to present conditions. There's now a wild card in the climatic deck, observes M.I.T. atmospheric scientist Kerry Emanuel: global warming. Over coming decades, atmospheric pollution and the greenhouse effect are expected to heat not just the air but also the surface of the oceans, and it is the thermal energy of that water that fuels typhoons and hurricanes. As a rule of thumb, according to Emanuel, wind speeds increase 5 m.p.h. for every additional degree Fahrenheit of water temperature. By that formula, sustained winds in future hurricanes could conceivably top 200 m.p.h.

But even these storms, it should be noted, would look puny compared with the megastorm of unimaginable destructiveness that scientists have dubbed a "hypercane." Indeed, some meteorologists speculate that a runaway hypercane, triggered by the splashdown of a giant asteroid, may have been instrumental in wiping out the dinosaurs 65 million years ago.

What makes hurricanes? They are, in essence, just big wind machines that move heat from the equator to the poles. While they do this very efficiently, the same task could be performed by swarms of independent thunderstorms. It takes a certain amount of magic, in other words, to set a hurricane in motion. First, you have to make the thunderstorms, and then "you have to get the thunderstorms dancing," as Florida State University climatologist James O'Brien puts it. "You have to get them dancing in a big circle dance."

In Floyd's case, the dance started when a disturbance high in the atmosphere moved off the coast of Africa and out over the Atlantic. Fueled by the rise of warm, humid air (in places, sea surface temperatures measured a steamy 86[degrees]F), the disturbance very quickly spawned a brood of thunderstorms that coalesced in a slow-moving whorl known as a tropical depression. On Sept. 8, as its winds reached 40 m.p.h., Floyd became a tropical storm. On Sept. 10, when its winds topped 74 m.p.h., it became a Category 1 hurricane. A few days later, with winds approaching 155 m.p.h., Floyd very nearly became a Category 5 storm--the highest category of all.

Meteorologists all agree that the energy powering Floyd--making it bigger than the average hurricane--came from the warmth of the water below. The tropical North Atlantic this fall was unusually warm, as it was during the period of high hurricane activity from the 1940s to the 1960s. Then, between about 1979 and 1995, the tropical North Atlantic cooled, and hurricane activity slackened. Now, notes David Enfield, a researcher at the Atlantic Oceanographic and Meteorological Laboratory in Miami, temperatures in this sector of ocean appear to be trending up once more. Like other oceanographers, Enfield believes this is the result of a natural climate shift, as opposed to human-induced global warming.

While warm water may be essential to the making of a hurricane, it is not sufficient, Colorado State University meteorologist William Gray points out. Gray has pioneered a hurricane-forecasting system that folds in many factors, including the strength of stratospheric winds, large-scale changes in ocean circulation, the amount of rainfall in West Africa and swings between El Nino and La Nina conditions in the equatorial Pacific. Indeed, says Gray, one reason the 1999 hurricane season has been so active is that the latest La Nina has persisted for more than a year.

Why would El Nino (which warms Pacific waters) and La Nina (which cools them) affect hurricanes in an entirely different ocean? The explanation is simple. El Nino enhances the influence of high-level westerly winds that swoop across the Atlantic, decapitating developing storm clouds before hurricanes can spawn. La Nina, by contrast, favors a more easterly flow that allows these clouds to mature into towering turrets, gathering energy as they grow. Florida State's O'Brien and two of his students have recently established that the chance of two or more hurricanes hitting the Eastern U.S. stands at about 25% when El Nino is ascendant but jumps to 75% when La Nina reigns.

Because of the El Nino and La Nina effects, and all those other factors, figuring out what might happen to hurricanes in a warmer world is, well, complicated. "Anything that does happen will likely cause only small changes," says Gray. "And no one can say which way these changes will go." If global warming favors more El Nino and fewer La Nina events, for instance, then the distribution of hurricanes will undoubtedly shift. But while there will be more typhoons in the Pacific and fewer hurricanes in the Atlantic, the total number of major storms is likely to remain the same. Worldwide, scientists think, there will probably continue to be about 80 such events in any given year.

How strong will those storms be? That's harder to estimate, in part because a very big storm is in some ways its own worst enemy. "A hurricane has a noticeable cooling effect on the ocean," explains atmospheric scientist Kevin Trenberth of the National Center for Atmospheric Research (NCAR). Indeed, at a certain stage of its life cycle, a storm of a given size will stir up enough cold water to put a halt to its growth. At that point, scientists say, it has come into equilibrium. Maintaining that balance is especially hard, because if a hurricane stirs up too much cold water, it will weaken and die. This suicidal tendency no doubt helps account for the fact that Category 5 hurricanes are so rare. Indeed, only two have hit the U.S. during this century, among them the 1969 hurricane named Camille.

How was it that Camille managed to grow so powerful? One reason, says Emanuel, is the path that Camille chose. She (in those days all hurricanes were of the feminine persuasion) faithfully followed the meanderings of the "loop current," a tributary of the Gulf Stream. It wasn't that the loop current was any warmer than the surrounding water at the surface, notes Emanuel, but its warmth went much deeper. Result: Camille's winds stirred up warm water as opposed to cold, and thus retained their strength.

Suppose, for the sake of argument, that global warming does cause the intensity of hurricanes to increase to supercane proportions. How stable would such megastorms be? A hurricane packing 200-m.p.h. winds would be significantly more powerful than Camille, whose top sustained winds were in the 180-m.p.h. range. Such a supercane would be capable, certainly, of taking a catastrophic toll, but its winds would also presumably penetrate to greater depths. Long before making landfall, a supercane might stir up a lethal dose of chilly water. More intense storms, in other words, could prove to be exceedingly fragile entities.

Of course, there are other ways in which global warming might boost the power of hurricanes. It's possible, for instance, that in a warmer world hurricanes might tank up with a lot more rain, which would greatly increase the damage caused by flooding. In addition, storm surges could be expected to become a lot more lethal if, as many anticipate, global sea levels rise.

But as NCAR's Roger Pielke Jr. observes, it's really not necessary to concoct ways to make hurricanes any more threatening than they already are. With or without global warming, there are going to be some whoppers in our future, and unlike Floyd, many of these will prove to be megadisasters. For the days when a big hurricane could make landfall in sparsely populated places are fast disappearing, Pielke notes, and that alone is cause enough for worry.

--With reporting by David Bjerklie/New York and Dick Thompson/Washington

With reporting by David Bjerklie/New York and Dick Thompson/Washington